Abstract Both the haemolytic activity and the binding ability to immunoglobulin G(IgG) (Fc-binding ability) were comparatively assayed among human, bovine, mouse and guinea-pig (C1q. The haemolytic activity was measured by using the sensitized sheep erythrocytes with rabbit immunoglobulin M(IgM)- or IgG-haemolysin. The Fc-binding ability was assayed by using immune complexes made of rabbit IgG-antibody against human serum albumin as well as agglutination of latex particles coated with human, bovine or rabbit IgG (IgG-latex). The specific haemolytic activity was comparable with between bovine and mouse C1q, while those of guinea pig and human C1q were significantly lower than those of the others. Only the human and mouse C1q showed significantly positive agglutinating activity of human or bovine IgG-latex. In the case of the use of rabbit IgG-latex, each of these C1q gave much weaker agglutination. On the other hand, the ability of all these C1q to bind to Fc of immune complexes specifically was almost comparable. The discrepancy in specific activities between the haemolysis and the Fc-binding ability may suggest that these two biological activities are not always correlative and that these are independent biological phenomena.